Isotropic polymer bonded rare earth permanent magnets have been used in various advanced motors and electronic devices. With the miniaturization of motors and electronic devices, it is necessary to reduce the size of magnets used. To enable effective miniaturization and efficient energy or signal output, it is essential for these applications to demand magnets of high flux densities. Factors determining the flux density of isotropic polymer bonded magnets can be divided into two parts: the type of magnet materials used and the volume fraction of the magnet materials in these polymer bonded magnets.
The criteria for selecting the type of magnetic materials are strongly influenced by the operation conditions demanded by the given applications. The polymer binder used for making polymer bonded magnets must be able to provide sufficient mechanical strength to hold magnet powder together and maintain the designed shape specification up to the intended operation temperature and to sustain that operation temperature without softening, deforming or breaking. The magnetic materials must provide sufficient flux to sustain the desired properties at the operation temperature without substantial loss of magnetization. The flux aging loss of a magnet material provides an indication of the magnet material's stability to heat and protection from corrosive and oxidative environments, which can affect the magnetic materials' ability to retain magnetic flux over time durations at certain temperatures. The flux aging loss of a bonded magnet ultimately determines the magnet's utility in various applications, and should be minimized if the bonded magnet is to be used for high temperature applications. The oxidative or corrosive degradation of the constituent materials and change in overall magnetic properties should be minimized to enhance the utility of the bonded magnets.
The combination and amounts of the organic and magnetic materials should allow the desired properties previously discussed to be optimized and attained. The amount of magnet powder in the polymer-bonded magnets, typically stated as a mass or volume fraction, is determined by the polymer binder type, molecular weight of the polymer binder, and the methodology applied in order to combine said materials effectively. Depending on the molding method, various polymers are available for making isotropic bonded magnets. Compression molding, injection molding, extrusion and calendering are well-known means for producing polymer-bonded magnets in commercial quantities.
Compression-molded or compacted magnets allow magnets to reach high, desirable volume fraction (greater than 83%) required to achieve strong magnetic properties. Typically, thermosetting polymers, such as, epoxies, phenolics, and other crosslinkable resins with their respective curing agents are used with the ideology of producing magnets that will less affected by heat or chemical attack than the non-coated powders. These materials are initially low molecular weight substances that can be easily applied as coatings for the magnetic powders. The components can be molded and cured to produce magnets that are resistant to high temperatures (typically, not much greater than 250° C.) and chemical solvents. The extent of crosslinking or the crosslink density of the thermosetting binder governs the coating's overall resistance to oxidation and corrosion as well as the mechanical strength of the final magnet.
At high loadings of solid filler the oxidation potential of the magnetic powders increases and becomes deleterious to the magnetic properties because of the low degree of organic protection, where the industry term “loading” refers to the proportion of magnet powder in the final magnet product. Chemical additives are introduced into the bonded magnet system in order to alleviate oxidative effects on the metallic filler particles. U.S. Pat. No. 5,888,416 to Ikuma et al. discloses the use of various chelating agents and antioxidants in the rare-earth bonded magnets of polyphenylene sulfide (PPS), nylon 12 (polyamide), and polyethernitrile (liquid crystal polymer) thermoplastic binders for use in extruded magnet compositions. U.S. Pat. No. 5,395,695 to Shain et al. discloses incorporating successive layers of an antioxidant, an epoxy novolac resin, and polystyrene onto the magnet material for improvements in oxidation resistance, with an emphasis on the sequential layering of the components. Xiao et al. and Guschl et al. describe the benefits of incorporating an aminosilane coupling agent onto the powders within a polyphenylene sulfide binder. See J. Xiao and J. U. Otaigbe, “High Performance, Lightweight Thermoplatic/Rare Earth Alloy Magnets,” Mat. Res. So. Symp. Proc., 577:75-80 (1999); P. C. Guschl, H. S. Kim, and J. U. Otaigbe, “Effects of a Nd—Fe—B Magnetic Filler on the Crystallization of Poly(phenylene sulfide),” J. Appl. Poly. Sci., 83:1091-1102 (2002). However, the results disclosed in these references were based solely on magnets with powder loadings on the order of about 80%, which is lower than those achievable in compression-molding magnets (on the order of about 90% or more). U.S. Pat. No. 4,876,305 to Mazany (“Mazany”) describes the application of a combination of aminosilanes and epoxysilane coupling agents with epoxy resins for oxidation resistance, comparing oxidation rates to treated and non-treated samples. The concentrations of magnetic material in the magnets disclosed in Mazany were fairly low, the magnetic properties of the resultant magnets, e.g., the flux aging loss, are not considered relevant.
U.S. Pat. No. 5,087,302 to Lin et al. discloses a process where an organotitanate is added to coarse Nd—Fe—B powders during a milling step to produce sintered magnets with improved magnetic remnance, coercivity and oxidation resistance. However, since the milled magnet powder-organotitanate mixture is subjected to a high-temperature degassing technique in an inert atmosphere in order to manufacture the sintered NdFeB magnets, the organotitanate is removed or “degassed” from the metallic powders.
Recently published coupling agents that have been utilized in bonded magnet systems are the organotitanates and organozirconates. Several Japanese patents describe the use of these agents and NdFeB powders with mainly nylon 12 resin, epoxy resins, PPS resin, and other such thermoplastic or thermosetting binders. See, e.g., JP-03165504 to T. Hitoshi et al.; JP-03222303 to M. Yoshihiko; JP-04011701 to M. Yoshihiko; and JP-04257203 to T. Hitoshi et al. These disclosures are directed to applications to injection-molding and extrusion-produced bonded magnets, because the specification of the material types and compositions disclosed therein fall well below the magnetic powder loadings disclosed in the present invention. Chen et al. disclose that a diaminoethylene-based titanate incorporated into a NdFeB-epoxy-bonded magnet system improved the bonding of the components and overall specific density of the magnet. See Q. Chen, J. Asuncion, J. Landi, and B. M. Ma, “The Effect of the Coupling Agent on the Packing Density and Corrosion Behavior of NdFeB and SmCo Bonded Magnets,” J. Appl. Phys., 85:8:5684-5686 (1999). However, no mention is made of the effects of the titanate on the flux aging loss of the magnet material or of the method in which the titanate was incorporated into the system. The present invention provides a more effective technique for protecting rare earth-transition metal-boron magnetic materials through a performing liquid coating procedure on the rare earth-transition metal-boron magnet powders. The present invention is applicable, for example, to compression-molding magnets.